Multiple bit rate (MBR) video encoding is a modern compression technique useful for delivering video over networks with time-varying bandwidth. MBR codecs (encoder/decoder systems) are used, for example, to provide video over the internet, and are also critical on mobile wireless networks in which the bandwidth available to a user changes dramatically over time. The 3GPP standards organization, for example, is adopting a MBR strategy as a standard for all High Speed Downlink Packet Access (HSDPA) terminals, and this strategy underlies the proprietary streaming formats from the leading vendors which provide streaming video. MBR video encoding techniques are useful because the bit rate of a video signal must be able to adapt to the changing network conditions while gracefully adjusting quality.
In particular, MBR video encoding techniques typically provide for such adaptability to the network conditions by creating a plurality of video sequences (or “copies”), each generated from the same video source material, and having a common set of switching points whereby a video system can switch between the copies. Thus, whenever network conditions change, the playback mechanism advantageously streams the copy that best matches the available bandwidth. Strategies for switching seamlessly between two or more of such video copies having different bit rates and common switching points are conventional and well known to those of ordinary skill in the art.
More specifically, in a typical MBR video system realization, several copies of the same video sequence are pre-encoded at different bit rates, and the playback system selects which video sequence to display from frame to frame. Only certain frames are valid “switching points” in which the decoder can start receiving a different stream and still recreate sensible video. However, since frames which are valid switching points must be encoded with use of far less efficient coding techniques than most other frames, there is an inherent tradeoff between the frequency of available of switching points and the overall efficiency of the multiple video encodings. (Typically, switching points must be encoded with use of a technique known as “intra-coding” rather than the technique of “inter-coding” used for most other frames. Each of these are fully familiar to those of ordinary skill in the art.) Frequent switching points are critical when the available bandwidth fluctuates rapidly (such as it does in wireless channels, for example), while the efficiency of a video encoding directly effects the overall video quality achievable at a given bit rate.